Mysterious Shadows - from The QuakeLab

The QuakeLab - http://www.in2nett.com/stevefu/QuakeLab

        TEXTURES:

        Your choice. Up to you. Whatever you want. Except CLIP, WATER, LAVA,
        or any such special textures. You won't see the thing creating
        the shadow anyway, so it really doesn't matter.

        However, you will need to use a SKY texture for this effect.

        ENTITIES:

        light

        WHAT TO DO:

        Make a room and texture the ceiling with a SKY texture. Copy the
        roof, lower it substantially, but keep it lined up with the
        ceiling. This copied ceiling brush is what the player will see
        in the game, so keep this in mind - when you build rooms next to
        the area containing this effect, you will want to take care that
        the height of the adjoining rooms makes sense in conjunction with
        the height of the sky in the room with this effect.

        Place the set of brushes (eg. the Quake logo) that will create the
        shadow between the real ceiling and the copied ceiling. Place a
        light in the same gap, but put it above the shadow-making brushes
        so that you actually make a shadow. :)

        The more space you have between the shadow-making brushes and the
        light, the smaller and more focused the shadow will be.

        Set the light's 'light' field to about 400 - this will depend on how
        large the room is.

        That's it. Keep in mind that only static brushes that are a part of
        the "worldspawn" will generate shadows. Entities DO NOT create
        shadows, not even func_walls, func_trains, func_plats, etc. 

        OPTIONS:

        This lighting effect can be used to create interesting visual clues
        in your levels, or just to add more aesthetic elements.